1. Technical Field
The present invention relates generally to light emitting diode (LED) arrays and the assembly thereof, and more particularly, but not exclusively, to such created in a custom fashion using primarily mechanical processes.
2. Background Art
A light emitting diode (LED) is essentially a PN junction semiconductor diode that emits light when current is applied across positive and negative leads, terminals, or connectors. By definition, the LED is a solid-state device that passes current without a heated filament, and thus is inherently more reliable. The materials used in the manufacture of the LED determine the color of the light produced. A clear epoxy resin is commonly used to encapsulate the LED or an assembly of LEDs to protect the dies and the electrical interconnections to them, and to allow light to pass out of the assembly.
The LED is highly efficient (˜90% efficient) for the conversion of electrons into photons. For comparison, incandescent lighting is roughly only 10% efficient, with 90% of the provided energy being converted to heat, and fluorescent lighting is only approximately 50% efficient. Having such high efficiency, technologies based on the LED are viewed as promising to help meet our future energy reduction goals.
In the operation of a LED the luminous intensity is roughly proportional to the amount of current that is supplied, and the higher the current the greater will be the light intensity produced, subject to the design limits of the device and the materials used. The amount of light emitted from an LED is quantified by a single point, on-axis luminous intensity value (Iv) and LED intensity is specified in terms of millicandela (mcd). In contrast, the light produced by incandescent lamps is usually quantified with a value for mean spherical candlepower (MSCP). These values for LEDs and incandescent lamps are not comparable.
In general, individual LED chips or dies are designed to operate around 20 milliamps (mA). Care must be exercised, however, as the operating current often must be limited relative to the amount of heat in the application. For example, a multiple-chip-in-a-package LED device incorporating multiple wire bonds and junction points will obviously produce more heat and thus be more affected by thermal stress than will a single-chip-in-a-package LED device. Similarly, LEDs designed to operate at higher voltages are subject to greater heat. Important design objectives for LEDs therefore usually include providing for long-life operation at optimum design currents and providing adequate heat dissipation as a defense against thermal degradation.
Presently, solders and soldering processes are commonly used to make the interconnections between a LED device and its sources of power and ground. The use of solders and higher temperature soldering processes, however, are rife with problems. These materials and processes have always had certain disadvantages, and a number of new trends in the electronics industry as well as newly emerging applications for LEDs are revealing or exacerbating other disadvantages, especially for arrays and other assemblies containing many LEDs.
One set of such disadvantages relates to solder materials. Tin/lead type solders (e.g., Sn63/Pb37) have been widely used since the earliest days of the electronics industry. Unfortunately, both tin and especially lead have serious chemical disadvantages. For these two metals, mining the ores, refining those ores, working with the refined metals during manufacturing, being exposed to substances including these in manufactured products, and disposing of the products at the end of their life cycles are all potentially damaging to human and animal health and to the environment.
Recently human health and environmental concerns about tin/lead type solders have resulted in the Directive on the Restriction of the Use of Certain Hazardous Substances in Electrical and Electronic Equipment (commonly referred to as the Restriction of Hazardous Substances Directive or RoHS) in the European Union. This directive restricts the use of six hazardous materials, including lead, in the manufacture of various types of electronic and electrical equipment. This directive is also closely linked with the Waste Electrical and Electronic Equipment Directive (WEEE) 2002/96/EC, which sets collection, recycling, and recovery targets for electrical goods. Together these directives are part of a growing world-wide legislative initiative to solve the problem of electronic device waste.
To some extent the electronics industry has always been searching for a practical substitute for tin/lead type solders, and legislative initiatives like those just noted are now motivating a number of changes. Today a common substitute for tin/lead type solders are SAC type solder varieties, which are alloys containing tin (Sn), silver (Ag), and copper (Cu). But this is merely a compromise. Mining, refining, working during manufacturing, exposure from manufactured products, and disposal are still all issues for tin, silver, and copper. Furthermore, SAC solder processes are prone to other problems, such as the formation of shorts (e.g., “tin whiskers”) and opens if surfaces are not properly prepared. It follows that the undue use of some materials, like those in solders, are generally undesirable in electronic assemblies, including LED assemblies.
Another set of disadvantages in the solder-based assembly of electronic products is the high temperature processes that are inherently required. The use of heat on and around many electronic components has always been undesirable. As a general principle, the heating of electronic components increases their failure rate in later use and beyond a certain point outright destroys such components. Tin/lead solders melt at moderate temperatures relative to the thermal limits of traditional materials used in electronics manufacture, and their use has generally been tolerable for many components. This is not always the same for SAC type solders, which melt at much higher temperatures (e.g., ˜40° C. or hotter). When SAC type solders are used the likelihood of component damage is much higher, resulting in assemblies that fail during post-manufacturing testing as well as in-the-field failures. Additionally, generating and managing the heat during manufacturing have increased energy, safety, and other costs. It therefore follows that the undue use of heat-based manufacturing processes, like soldering, is also generally undesirable in electronic assemblies.
Increasingly yet another set of disadvantages in the solder-based assembly of electronic products is one related to the “adding” of materials. When a material, like solder, is added between two components to hold them together the additional material inherently has to occupy some space. Solders contain higher density metals, thus increasing the ultimate weight of electronic products. The use of liquid-state materials, like liquid stage solder during manufacturing, often requires designing in additional space around leads, terminals, and connection pads to account for the ability of the liquid to flow easily and to potentially short to other leads, terminals, pads, etc. Liquid solders have high surface tensions and effects from this also usually require major design consideration. These are all factors that can require consideration as designers increasingly strive to miniaturize electronic assemblies. Accordingly, it further follows that the undue use of any additional material in manufactured assemblies and in manufacturing processes, again like solder, is generally undesirable in the resulting electronic assemblies.
In addition to the noted disadvantages in the solder-based assembly of electronic products, generally, there can be additional problems in particular in the solder-based assembly of LEDs. For example, in an LED the soldering process can be difficult because the ideal substrates for thermal degradation protection are typically good thermal conductors, purposefully being used because they have a high thermal capacity that will help keep the LED assembly within in desired temperature range during operation. This creates a significant challenge for solder assembly of LED packages, however, because the package must be raised to an even more elevated temperature to create reliable solder joints and the necessary temperatures can then degrade or damage the encapsulant used in the manufacture of the LED package. Moreover, exposure to certain cleaning chemicals may attack the LED surface and cause discoloration.
FIGS. 1-2 will help to illustrate some of the above points, as well as help to introduce some additional points.
FIG. 1 (background art) is a cross-section side view of a typical LED package 10 which may be used in conventional LED assemblies. The LED package 10 includes a conventional LED die 12 that has a first connector 14, a second connector 16, a P-layer 18, a P-N junction 20, and an N-layer 22. The first connector 14 here is electrically connected to a first terminal 24 with a conductive lead 26 and the second connector 16 is directly electrically connected to a second terminal 28.
A body 30 is further provided that fills multiple roles. For example, the body 30 physically holds the other elements of the LED package 10 in fixed relationships. This serves to protect the internal elements of the LED package 10 (i.e., the LED die 12 and the conductive lead 26), to place and retain the externally communicating elements of the LED package 10 where needed, and generally to facilitate handling of the LED package 10 when mounting it into an electronic assembly or a larger LED assembly. The body 30 also serves to optically pass the light wavelengths that the LED die 12 emits. For this the body 30 particularly has a face 32 where light from the LED die 12 is primarily emitted from the LED package 10. The body 30 may also serve to conduct heat away from internal elements of the LED package 10. Historically this thermally conductive role has usually not been an important one, but that is now changing, especially for emerging high power LED applications. In view of all of these roles, the body 30 of the LED package 10 is typically of a single plastic material, with glass, quartz, or hybrids of materials sometimes also being used.
FIG. 2 (prior art) is a cross-section side view of a conventional LED assembly 50 that includes the LED package 10 of FIG. 1. The LED assembly 50 here is oriented as it is typically manufactured and as it is often used, that is, with the light emitting face 32 of the LED package 10 oriented upward.
In this orientation the LED assembly 50 is now discussed as generally being “built” from the bottom up. An electrically insulting substrate 52 is usually provided, if for no other reason than to physically support an anode trace 54 and a cathode trace 56 as shown. However, optional elements may also be provided in a sub-region 58 below the substrate 52. For example, if the substrate 52 is the top most non-conductive layer of a printed circuit board (PCB), other layers may also be present in this sub-region 58 (e.g., a ground plane or “reverse side” feature if the printed circuit board is double sided).
For some emerging applications a feature that may particularly be present in the sub-region 58 below the substrate 52 is a heat spreader. The substrate 52 will typically serve to some extent to transfer heat, but it may not be optimal for that. To clarify, the role of a heat sink (which many in the art are more familiar with) and that of a heat spreader are different. Although these elements operate similarly to some extent, a head sink is optimized to remove thermal energy from a particular location, typically a point or small location, whereas a head spreader is optimized to distribute and equalize thermal energy across an area or large location.
Continuing with FIG. 2, the anode trace 54 and the cathode trace 56 are located above the substrate 52. Again, the common PCB serves as a useful example here. In a PCB the substrate 52 is usually an electrical insulating material, the traces 54, 56 are copper foil, and the necessary pattern of the traces 54, 56 on the substrate 52 is achieved with silkscreen printing, photolithography, milling, or some other suitable process.
Of particular interest here is the next higher feature in the LED assembly 50, a set of solder pads 60. These electrically connect the anode trace 54 to the first terminal 24 and the cathode trace 56 to the second terminal 28 of the LED package 10. The solder pads 60 also physically connect the LED package 10 to the rest of the LED assembly 50, thus holding the LED package 10 in place.
The possible materials in the solder pads 60 have already been discussed elsewhere herein and are legend. It should further be observed here, however, that the solder pads 60 inherently add an additional level or displacement layer 62 to the overall LED assembly 50. In applications where the overall thickness of the LED assembly 50 is critical, this displacement layer 62 can be a concern and minimizing or eliminating it can then be an important goal.
FIG. 2 also stylistically shows thermal flow paths 64 out of the LED package 10 and into the LED assembly 50. As can be seen here, much of the thermal energy produced by the LED package 10 passes through the solder pads 60, with the majority of it flowing through the second terminal 28 and the cathode trace 56. In some applications this thermal flow can cause serious problems. For instance, if too much heat builds up in the LED package 10 it may be damaged internally. The solder pads 60 tend to be thermally conductive, but they nonetheless lengthen and complicate the primary paths that thermal energy must travel to exit the LED package 10. Furthermore, since the flow of thermal energy in the structures of the LED package 10 and in the overall LED assembly 50 are not instantaneous, localized heating can result (e.g., in the region at the second terminal 28 of the LED package 10 in FIG. 2). This can thermally stress the LED die 12, the LED package 10, and the LED assembly 50. In extreme situations this can cause separation of a solder pad 60 from the first terminal 24, the second terminal 28, or from a trace 54, 56 and such stress can even result in a fracture of the body 30 of the LED package 10.
In FIG. 2 the thermal flow paths 64 out the top and sides of the LED package 10 are minimal (as stylistically depicted with lesser weight arrows). There is little that can be done with respect to the top of the LED package 10, since the face 32 here needs to emit the light produced. But the sides of the LED package 10 would appear to be another matter. Unfortunately however, the solder pads 60 tend to interfere with what can be done here. Having the sides of the LED package 10 open (as shown in FIG. 2) is desirable when the LED package 10 is soldered into the LED assembly 50, especially in surface mount device (SMD) embodiments of the LED assembly 50 where surface tension effects of the liquid solder are relied on to help position the LED package 10. But after soldering, wick regions 66 in the solder pads 60 (also caused by surface tension effects when the solder is liquid) usually remain and can interfere with adding a thermal conductor to the sides of the LED package 10 once it is in the LED assembly 50.
In summary, the use of solder materials, the use of heat-based soldering manufacturing processes, the undue addition of solder material to manufactured assemblies and these and additional problems particular to the solder-based assembly of LEDs are all generally undesirable.